1. Field of the Invention
The present invention relates to light dispersing devices and, in particular, relates to a thermally compensated light dispersing device utilizing a diffraction grating.
2. Description of the Related Art
Dispersing devices are often used in science and industry to separate beams of electromagnetic radiation, or light beams, according to wavelength. In particular, such devices are adapted to receive a polychromatic input light beam having a relatively large combined spectral bandwidth and convert the input beam into a plurality of constituent output light beams having relatively narrow bandwidths. The output beams exit the dispersing device with wavelength dependent directions so as to allow subsequent processing of individual wavelength components of the input beam.
One type of light dispersing device is a diffraction grating. The typical diffraction grating comprises a planar substrate and a relatively thin contoured layer formed on the substrate such that a contoured outer surface of the contoured layer defines a plurality of narrow grooves or slits. In the case of a reflection type grating, the contoured layer is formed of material having a high reflectivity so that light impinging the contoured surface is reflected therefrom. In the case of a transmission type grating, the contoured surface is provided with a low reflectivity and the substrate is formed of light transparent material so that light impinging the contoured surface is transmitted through the grating.
Thus, when a polychromatic incident light beam having a generally planar wavefront impinges on the contoured surface of the typical grating, a wavelet having a spherical wavefront is emitted from each of the grooves of the grating. As the wavelets travel away from the grating, they overlap and interfere with each other so as to provide a plurality of substantially monochromatic diffracted beams having wavelength dependent directions. Since the spectral bandwidth of the output beams is dependent on the number of grooves of the grating and since the contoured surface of the typical grating usually comprises a high density of grooves per unit length, the output beams are often provided with relatively narrow spectral bandwidths.
Mathematically, the dispersing characteristics of the diffraction grating are defined by the equation
s(sin xcex8ixc2x1sin xcex8m)=mxcexxe2x80x83xe2x80x83(1)
wherein s is the groove spacing of the grating, xcex is the wavelength of the output beam, xcex8i is the incident angle of the input beam with respect to a line normal to the plane of the grating, xcex8m is the outgoing or diffraction angle of each output beam with respect to the line normal to the grating, m, otherwise known as the order of interference, can take any integer value, and wherein the plus sign applies to reflection-type gratings and the minus sign applies to transmission type gratings. Furthermore, the wavelength xcex of the output beam is defined by the equation                     λ        =                              λ            f                                n            medium                                              (        2        )            
wherein xcexf, otherwise known as the free space wavelength, is the wavelength of the diffracted beam in a vacuum, and nmedium is the index of refraction of the medium in which the diffracted beam is traveling.
However, diffraction gratings known in the art often provide unstable dispersing characteristics that vary in response to a changing temperature. In particular, the groove spacing s is often affected by a change in temperature due primarily to the substrate of the grating having a non-zero coefficient of thermal expansion (CTE). Because the groove spacing is defined by the shape of the contoured surface and because the contoured layer is physically in contact with the substrate, the groove spacing s can change in response to expansion or contraction of the substrate. Since the groove spacing affects the dispersive characteristics of the grating according to equation (1), the dispersing characteristics of the typical diffraction grating may vary in a temperature dependent manner.
Another problem associated with typical diffraction gratings is that it is usually difficult to realize their theoretical maximum efficiency. In particular, the efficiency of a reflection-type grating is increased when the input and output beams are substantially aligned with each other. However, additional constraints imposed by the necessity of positioning additional elements adjacent the grating require the output beams to be angularly separated from the input beam with relatively large angles defined therebetween.
For example, in some configurations, a lens is often required to be positioned in the path of the input beam prior to the grating so as to collimate the input beam. Furthermore, spatial constraints may require the input lens to be positioned adjacent the grating. To prevent the input lens from distorting the output beams, the output beams are required to define relatively large angles with respect to the input beam. Consequently, the typical grating often provides less than optimal performance.
A prism is another type of dispersing device that is often used to extract narrow-band light beams from a polychromatic input beam. The typical prism utilizes non-parallel refracting input and output surfaces in conjunction with a transparent medium having a wavelength dependent index of refraction to provide the desired dispersing characteristics. Light entering and exiting each refracting surface of the prism is refracted according to Snell""s Law of refraction which can be expressed mathematically by the equation:
ni sin xcex8i=nr sin xcex8rxe2x80x83xe2x80x83(3)
wherein ni is the index of refraction of an incident medium, xcex8i is the incident angle of the incident beam defined with respect to a line perpendicular to the refracting surface, nr is the index of refraction of the refracting medium adjacent the incident medium, and xcex8r is the refracted angle of the refracted beam. Because the index of refraction of the prism is dependent on the free space wavelength of light traveling therethrough, light exiting the output surface does so with a wavelength dependent direction. However, since the index of refraction of the typical prism usually varies with a change in temperature in a substantial manner, the dispersing characteristics of the prism are also relatively unstable and vary in response to a change in temperature.
Another type of dispersing device combines a diffraction grating and a prism so as to form what is otherwise known as a xe2x80x9cgrismxe2x80x9d. One advantage of the grism is that it provides convenient mounting surfaces for facilitating alignment of the grism within an optical system. Furthermore, as described in U.S. Pat. No. 5,652,681 to Chen et al., the grism can be adapted to provide improved resolving power, such that the device is able to disperse finely spaced wavelength components with increased angular separation over a larger spectral range. However, because the groove spacing of the typical diffraction grating and the index of refraction of the typical prism are both adversely affected by a change in temperature, the dispersive characteristics of the typical grism are especially sensitive to a change in temperature.
From the foregoing, therefore, it will be appreciated that there is a need for an improved light dispersing device with dispersing characteristics that are more stable with respect to temperature. Furthermore, there is a need for a dispersing device with increased throughput efficiency.
The aforementioned needs are satisfied by the present invention which, in one aspect, comprises a dispersing apparatus having an input face, a diffracting element, and an output face. The input face refracts light at angles which vary with temperature. The diffracting element compensates for these angular variations such that light exits the dispersing apparatus refracted at angles that are substantially independent of the angular variations of the input face.
In one embodiment, light entering the dispersing apparatus is a polychromatic light beam and said light exiting the dispersing apparatus is a plurality of narrow-band output light beams. The input and output faces are preferably formed by a prism, and said diffracting element preferably has a diffracting face. Furthermore, the diffracting element is preferably a diffraction grating juxtaposed with the prism.
In another aspect of the present invention, a light dispersing apparatus disperses a polychromatic input light beam into a plurality of narrow-band output light beams such that each output beam exits the dispersing apparatus with a wavelength dependent exit angle. The apparatus comprises a prism formed of a transparent medium having an index of refraction np. The prism has a mounting surface, an input surface for receiving the input beam, and an output surface for providing said output beam. The prism provides first dispersing characteristics that vary in response to a change in temperature. The apparatus further comprises a diffraction grating coupled to the mounting surface of the prism. The diffraction grating provides second dispersing characteristics that vary in response to a change in temperature. The variations in the first and second dispersing characteristics cooperate with each other so as to substantially reduce variations in the exit angle of each output beam in response to a change in temperature.
In a further aspect of the present invention, a dispersing apparatus comprises an input face for refracting said input light beam and a reflecting face for reflecting said input light beam subsequent to said refracting of the input light beam by the input face. The apparatus further comprises a diffracting face for diffracting said input light beam subsequent to said reflecting of the input light beam by the reflecting face. The diffracting face provides at least one diffracted beam. The apparatus further comprises an output face for refracting the at least one diffracted beam so as to provide the at least one output light beam.
In one embodiment, the input light beam enters the diffracting face along a path that is substantially juxtaposed with the path of at least one diffracted beam exiting the diffracting face so as to increase the diffraction efficiency of the diffracting face. The input light beam entering the diffracting face partially overlaps the at least one diffracted beam exiting the diffracting face so as to increase the diffraction efficiency of the diffracting face. The reflecting of the input beam at the reflecting surface occurs as a result of the input beam undergoing total internal reflection.
In yet another aspect of the present invention, a method comprises directing a beam of light through a medium along a first path to a diffracting element. The beam is then diffracted through the medium along a second path. After such diffracting, the beam is refracted into a different medium to direct the beam along a third beam path. The first and second beam paths are then altered without substantially altering the third beam path. In one embodiment, the first and second beam paths are altered by changing the temperature of the medium.
Another aspect of the present invention is a method which comprises directing a beam of light through a medium along a first path to a reflecting surface. The beam is then reflected for propogation through the medium along a second path that extends from the reflecting surface to a diffracting element. The beam is then diffracted for propagation through the medium along a third path. After such diffracting, the beam is refracted into a different medium to direct the beam along a fourth beam path.
In one embodiment, diffracting the beam through the medium along a third path comprises diffracting the beam through the medium along a third path such that the third path is substantially juxtaposed with the second path. In one embodiment, the method further comprises altering the first, second, and third beam paths without substantially altering the fourth beam path. In one embodiment, altering the first, second, and third beam paths comprises changing the temperature of the medium.